Figure shows the plane wavefront $\mathrm{XY}$ of a parallel beam of light incident to a thin convex lens and rays coming out through the thin convex lens is shown to be concentrated at second focus point. To draw a wavefront corresponding to these rays, the circles should be drawn by accepting second focus as a centre. The arc $X^{\prime} Y^{\prime}$ of such circles is shown in the figure.

This arc is the wavefront corresponding to the refracted rays at some instant.

Here the distances from $A$ to $a$ and $C$ to $c$ are greater then B to $b$. So the light has taken more distance in the lens going for $B$ to $b$ and the velocity of light in the material of lens is less. Hence, the point $b$ on the wavefront lags behind $a$ and $c$.